RECENT ADVANCES IN OUR KNOWLEDGE OF ECDYSTEROID BIOSYNTHESIS IN INSECTS AND CRUSTACEANS

Many advances in arthropod sterol metabolism, as it relates to ecdysteroid biosynthesis, have been made in the past 10 yr. The sterol 24-dealkylation system, present in the midgut of many species, is entirely absent in some members of at least four insect orders and the prothoracic glands of Manduca sexta. While Delta(5)-sterols are precursors in arthropod ecdysteroid biosynthesis, the Delta(7)-sterol, lathosterol, is the precursor in at least some plants. 7-Dehydrosterols are converted directly into ecdysteroids by insect prothoracic glands and ovaries, as well as crustacean Y-organs. Conclusive evidence for many potential mechanisms of sterol 7,8-dehydrogenation and conversion of the Delta(5,7)-sterols to 2,22,25-trideoxyecdysteroids is still lacking, though circumstantial evidence for a 5,6-epoxy-Delta(7)-sterol is discussed. The physiological relevance of 3-dehydroecdysteroids in some insects has only recently been appreciated. The sterol 7,8-dehydrogenating enzyme, the sequential terminal hydroxylases and ecdysone 20-monooxygenase all exhibit properties of cytochrome P-450 enzymes. The lack of tissue and substrate specificity reported for the terminal hydroxylations of exogenous substrates suggests that non-specific hydroxylases may be involved. Extensive pharmacological investigations of ecdysone 20-monooxygenase continue, and efforts toward the isolation and sequencing of this enzyme are underway. Developmental correlations suggest that the sterol 7,8-dehydrogenase and terminal hydroxylases in Manduca prothoracic glands are not regulated by prothoracicotropic hormone, in contrast to the utilization of 7-dehydrocholesterol. In crustaceans, the terminal hydroxylases are influenced by molt inhibiting hormone.